* nesting function
* hidden pointer as record
* array
* int, string
* kinds of control struct

* include: [Identifiers, Comments, Integer literal, String literal]
* reserved words: [while, break, let, in, end, function, var, type, array, if, then, else, do, of, nil]
* interpunction: , : ; ( ) { } [ ] . + - * / = <> >= <= < > & | :=
* comment(can be nested): /* here  /* is */ content */

* `xxx` means strong reserved words xxx
*  {x}  means probility empty
*   E   means empty string
*  //   means about note
*   $   means such that

decs -> {dec}
dec  -> tydec
     -> vardec
     -> fundec

   typedec -> `type` type_id = ty
        ty -> type_id
           -> {typefields}
           -> array of type_id

typefields -> E
           -> id: type_id{, id: type_id}

    //bulit-in type: `int`, `string`
    //record type: type Node = {key: int, next: Node}
    //array type: type intArray = array of int

vardec -> `var` id := expr
       -> `var` id: type_id := expr
       
   //if short type_id then id's type decided by invalid expr's type

funcdec -> function id(params) = expr
        -> function id(params): type_id = expr

//if short type_id then return void

* `let ...vardec | typedec | funcdec... in expr end` begin vardec end in `end`
* function params only live in function-body
* nest scope: Tiger allow visit outer scope var if def
* namespace: 1. Type's($ Node.int) 2. Var | Function ($ Node(), Node+1)
* rename: the lastest def always worked

lvalue -> id
       -> lvalue.id
       -> lvalue[int-expr]
// lvalue is a memory addr
// var, params, record-filed, array-index can be lvalue

* lvalue:
  when a lvalue in a expr, eval is the value in the mem-addr
* void:
  some expr no eval $ process call, :=, if-then, while, break,
  e.g. (a := b) + c is mistake semant
* nil:
  nil is specilly belongs record type value.
  if record v is nil, $ v.a is a running-time error.
  it requeried nil must be assigned in a certain type.
* sequence:
  like this (expr; expr; expr)
  eval all the expr in order and choose the last expr(if there is) as the whole expr's value.
  specially, () means void, `let ... in %nothing% end` also void
* literal:
  decimal const int value
  string  const 
  negative const there is a minus sign before the number
* arithmetic:
  int-expr op int-expr $ op [+, -, *, /] eval a int expr
* compare:
  expr op expr $ op [=, <>, <, >, >=, <=] 
  true => 1, false => 0.
  = and <> op can also apply in record and array.(cmp them refrence)
  string also can be cmp
* bool expr:
  expr op expr $ op [&, |]
  0 means false, not zero means ture
* operotor proprity:
  proprity is uncertain due to comliter's uncertain
  but ((`expr`) op expr) certain first eval nest-in expr
* record new:
  record_type_id{filed_id = expr, ...}
  or nil alloc memory but no init
* array new:
  array_type_id[int-expr] of ii-expr
  ii-expr is the init value
* assign:
  when record or array assign they are equally point a same mem.
  $ they are alloc in heap. alive all the program

* function print(s: string) //print s to the stdout
* function flush() //empty the stdout cache
* function getchar(): string //from stdin read a char, if EOF return '\0'
* function ord(s: string): int //print the first char ascii number in s, if s is empty return -1
* function chr(i: int): string //inverse the prior function.
* function size(s: string): int // return length of s
* function substring(s: string, begin: int, length: int): string //sub from begin and length 's string
* function concat(s1: string, s2: string): string //link s1 and s2 get a new string
* function not(i: int): int // eval logic reverse
* function exit(i: int) // exit the program by status code i

* in the test/ dir there are 50s tig file you can test and refer

* blog:[Tiger](https://github.com/whps/whps.github.io/issues/5)
* book: <Modern Compiler Implement in C>